Loose schooling. What to do when your horse thinks it is play time, not work time...
How big is your arena?
If it is possible to section a small area off inside your arena, then I find that helps enormously.
Have you done any ground work with your horse, something along the lines of listening to your body language, along the lines of natural horsemanship? Horses are constantly evaluating your body language, even if they don't appear to be looking at you, they are very much aware of what you are communicating to them.
If you can section off a smaller area safely, then one of the best and easiest ways to begin is to change direction frequently, this tunes your horse in, and allows you to control the pace easier, once you are changing direction, you should be able to begin to use your usual "aids" as on the lunge to ask your horse what is required of him.
To ask your horse to change direction, position yourself towards the front of the horse (never putting yourself in danger) and make yourself "big" I.e. bold body language , hands raised but not threatening, and purposeful, once your horse turns, return to being "behind" him, I.e. your hips to his hips, as if lunging, then repeat to turn again.
It may take some practice, but once you have the basics established in a small area, it becomes far easier to apply the same principles once in a larger area.
My horse gets wound up by others nearby: If the other horses are galloping around, then it is even more important to gain your horses concentration, so again, frequent changes of direction and if possible transitions.
If you cannot reduce the size of the arena, then poles placed around the track, especially the side your horse chooses to show off, at intervals can also be useful, a set of trot distance poles, a set of canter distance poles and single poles around the arena are also useful to gain your horses attention.
Should you have the time, daily “schooling” in the arena can help take the excitement out of them, as it becomes a little more work time rather than play time.
I hope these ideas help and you can gain some good work from your horse, loose schooling is a great exercise for the horse as they have to rely on their own balance rather than balance on the rider/handler.